Variable transmittance optical filters may employ a variety of technologies to alter visible light transmittance. Generally, such filters may be switched between a state of higher light transmittance (faded or light state) to a state of lower light transmittance (dark state) with the application, removal or reduction of a stimulus such as UV light, temperature and/or a voltage. Examples include photochromics, electrochromics, thermochromics, liquid crystals or suspended particles. Some photochromic materials may darken in response to light, frequently ultraviolet light, and may return to a faded state when the UV light is removed or reduced. Some electrochromic materials may darken in response to application of a voltage, and may return to a faded state once the voltage is removed; alternately, some electrochromic materials may darken in response to application of a voltage of a first polarity, and fade when a voltage of an opposite polarity is applied. Some thermochromic materials may darken proportionately in response to a temperature increase—for example, the warmer the material, the darker it can become. The thermochromic material may return to a faded state when the temperature decreases. Liquid crystal materials and suspended particle devices comprise crystals or particles that alter orientation in response to application of a voltage. In the absence of a voltage, the crystals or particles are randomly oriented, and scatter incident light, thus appearing opaque, or transmit very little light. When a voltage is applied, the crystals or particles are aligned with the electric field, and light may be transmitted. Where the variable transmittance optical filter includes an electrochromic aspect, the variable transmittance optical filter may comprise electrical connectors for connecting the optical filter to a control circuit, the control circuit to provide power to the optical filter to effect an electrochromic color change.
Depending on the nature of the variable optical filter and its use, further attenuation of the transmitted light or solar energy may be desirable. Where the variable transmittance optical filter is used on the window of a vehicle, aircraft or building, reducing or blocking transmission of infrared light may be useful to control the heat gain, and reducing or blocking transmission of ultraviolet light may be useful to protect occupants in the vehicle or building. Where impact protection is desirable, inclusion of laminated glass (“safety glass”) in the window may useful.
U.S. Pat. No. 4,244,997 and US 2009/0303581 describe a laminated glass with a shade band and U.S. Pat. No. 7,655,314 describes a laminated glass with an interlayer comprising an IR blocking component, and a coloring agent to complement the yellow-green appearance of the IR blocking component, but does not address how the color may be manipulated in a window with variable light transmission in the visible range. Tinted glass in grey, bronze or green tones may also be used to attenuate the light transmitted through a window. Some tints may attenuate light approximately equally across the visible spectrum, and while this may be effective in reducing the overall glare, it may not provide for color “correction” to a neutral tone if a component of the laminated glass itself has a color, and additional color correction may be needed.
Some examples of windows of vehicles that may alter light transmission or opacity with electricity are known—the Magic Sky™ automotive sunroof is one example of automotive glass that switches from an opaque state to a transparent state with the application of electricity. The switchable layer (suspended particle thin film) is applied to the sunroof glass and connected to the vehicle's electrical system. U.S. Pat. No. 6,995,891 describes an electrochromic safety glazing comprising an electrolyte interlayer with a polymeric binder for lamination of the substrates.
Where the laminated glass has a variable transmittance component, the degree of light transmission in one or both of the faded and dark states may be too great, or of a distorted color. Previously, colour balancing of glazing products such as automotive sunroofs and architectural windows was accomplished by altering the chemical composition of the glass itself to provide the desired colour, or by including a coloured interlayer (e.g. PVB) in between two sheets of glass. Altering the colour of the variable transmittance filter is much more difficult because the materials used for producing the variable transmittance cannot easily be changed to different colors while maintaining all of the variable transmittance properties. For example, some variable transmittance filters are blue in colour, which may be suitable for some applications but not others. Currently, the color of the overall product is determined by the color of the variable transmittance filter, even if that color is not seen as the most desirable by customers and potential customers of the product. Inclusion of one or more additional visible light filters may further attenuate the transmitted light, but may also distort the color or exacerbate an already distorted color.